This invention relates to a water-based chloroprene polymer adhesive composition particularly suitable for bonding to substrates composite structures which can be subjected to postforming into useful shapes at elevated temperatures, e.g., in the neighborhood of 120.degree. C. or higher.
A composite structure such as, e.g., a multilayer laminate for decorative tabletops or countertops, often must be postformed at high temperatures in order to be fitted to the shape of the substrate. For example, the composite structure may be first bonded to the substrate at ambient or a moderately elevated temperature in a manner to leave a portion of the composite structure overhanging one or more edges of the substrate. Those portions are then bent at a higher temperature to cover the edges, and those bent portions are bonded to the edges, or both to the edges and to the underside of the substrate while still hot. As the thus produced assembly cools, stresses develop in the original composite structure which can cause warping and consequently separation of the laminate from the substrate. It is, therefore, very important to assure that the bond between the substrate and the laminate is capable of withstanding such elevated temperatures without failure.
Chloroprene is the common name of 2-chloro-1,3-butadiene. It is known to homopolymerize and copolymerize chloroprene to high molecular weight elastomeric materials, which often are referred to in the chemical industry as the neoprenes. The term "neoprene" will be occasionally used herein to designate a chloroprene polymer.
Neoprene adhesives are well known. Frequently they are formulated as contact adhesives, which are available in the form of cements, i.e., solutions in organic liquids. Water-based neoprene contact adhesive compositions also are known. However, the presently available water-based neoprene adhesives do not provide bond strengths comparable to those of neoprene cements. Further, those bonds usually cannot withstand temperatures in excess of about 100.degree. C., which are encountered when postforming and bonding composite structures, without substantial loss of strength.
Still, because of the fire hazards associated with solutions in organic liquids, as well as of the growing concern for the environment, it is no longer desirable to use solvent-based adhesives, and it is preferred to replace them to the extent possible with water-based adhesives.
It is known that polymerization of chloroprene at low temperature increases the crystallization rate of the polymer. See J. T. Maynard, J. Poly. Sci., vol. 18, p. 227 (1955). Neoprene cements usually contain polychloroprene made at a low temperature and have a high green bond strength. Those bonds retain sufficient strength in the high temperature postforming and bonding of composite structures.
Copolymers of chloroprene with comonomers which would retard crystallization, such as, e.g., 2,3-dichloro-1,3-butadiene, also are known. However, copolymers of chloroprene with 2,3-dichloro-1,3-butadiene are known to be deficient in their bonding strength and, therefore, must be formulated with additional special components such as, e.g., polyamines, epoxy compounds, and additionally tackifiers. The art discloses a number of special conditions required for preparing chloroprene copolymers having high bond strength, or special comonomers that would assure good bonding properties, or special additives required to improve the bonding properties of chloroprene copolymers. See, e.g., U.S. Pat. Nos. 4,130,528 to Chen and 4,463,110 to Perlinski et al.
There is a need in the industry for a water-based neoprene adhesive composition having high green bond strength, capable of substantially maintaining its bond strength at elevated temperatures, and capable of being formulated with conventional additives.